Ilex crenata Thunb. `Rotundifolia' split-root plants were grown for 3 weeks at root-zone temperatures of 30/30, 30/34, 30/38, 30/42, 34/34, 38/38 and 42/42. The 38 C root-zone temperature treatment was the upper threshold for a number of growth and physiological parameters. A portion of the root system grown at near optimum temperatures could compensate in terms of shoot growth for part of the root system exposed to supraoptimal root-zone temperatures up to the 38 C critical threshold. Higher root-zone temperatures did not affect photosynthetic rates or root:shoot ratios, but altered photosynthate partitioning to different stem and root sinks. Although no differences were found for total 14C partitioned to the roots, partitioning of the 14C into soluble and insoluble fractions and the magnitude of root respiration and exudation were influenced by treatment. Heating half of a root system at 38 C increased the amount of 14C respired from the heated side and increased the total CO2 respired from the non-heated (30 C) half. Exposure of both root halves to 42 C resulted in membrane damage which increased the leakage of 14C photosynthates into the medium.